The following discussion is not an admission that anything described below is common general knowledge or citable as prior art.
Aerators are used in immersed membrane systems, for example, to produce bubbles in water below a membrane module. The aerator may be in the form of a tube or other hollow body connected to a source of pressurized air, or another gas, and having holes to release the air as bubbles from the aerator body. The bubbles rise through the module, scouring the membranes to inhibit fouling. However, particularly when the module is immersed in mixed liquor or other types of wastewater, the aerator itself can become fouled. For example, during periods of time when the air supply is shut off for maintenance or other reasons, water may enter the aerator. When the air is turned back on, the water is blown out but some solids may stick to the surfaces of the aerator or be mechanically trapped in the aerator. These solids dry out and harden, and may attract or trap additional solids the next time the aerator is flooded. Eventually, the solids may accumulate to the point where they partially or completely block one or more of the holes. When this happens the aerator does not deliver a full flow of bubbles, and this allows the membrane modules to foul more rapidly.
U.S. Pat. No. 5,944,997 describes a method of cleaning fouled aerators in a membrane bioreactor. The aerators are flushed with a burst of liquid permeate, or with a burst of liquid permeate with entrained air bubbles. U.S. Pat. No. 6,245,239 describes a cyclic aeration system for submerged membrane modules. In this system, the air supply to an aerator is turned on and off frequently, for example every 20 seconds. While the air supply is off, an air supply pipe may be vented to intentionally flood the aerator. In this way, the solids are kept from drying out to reduce their tendency to plug the aerator. However, the venting requires additional valve operations, solids may still accumulate at the flooded water level, and solids such as fibers that are mechanically trapped can still accumulate in the aerator.